In letdown services, high performance letdown valves are conventionally employed to reduce a flow of a material from a high pressure to a lower pressure. One example of a system employing a letdown valve includes systems for carrying out a conventional pressure acid leaching (PAL) process or high-pressure acid leaching (HPAL) process. An example of such a process was developed at Moa Bay in Cuba in the late 1950s and is sometimes referred to as the Moa Bay process.
An example of a Moa Bay process comprises pulping ore to approximately 40% solids in a fluid, such as water, and then feeding the ore in the slurry form to various compartments of an autoclave. Sulfuric acid is fed into the autoclave to selectively leach materials from the ore slurry at elevated temperatures and pressure (e.g., about 250° C. and 3.62 MPa) to solubilize the various materials. As leaching progresses, the leached slurry is passed to one or more level-control vessels and to one or more letdown stages (i.e., a vessel such as a flash tank) through letdown valves.
The slurry passing through the letdown valves is typically a highly abrasive mixture of solids, liquids and gases flowing at high temperatures and pressures and may be traveling through the letdown valves at relatively high velocities (e.g., as high as mach 1 [343 m/s]). The highly abrasive make-up of the slurry, combined with the high temperatures, pressures and velocities typically result in accelerated wear to the letdown valves and associated components.
Furthermore, the slurry mixture tends to not be homogeneous, resulting in surging of the flow as the mixture exits the letdown valve. Such surges often cause low frequency vibrations in the piping and may result in splashing in the vessel, as well as forces on the plug head of the letdown valve. Splashing inside the vessel accelerates wear to the vessel and reduces the life of the vessel. Similarly, the forces on the plug head of the letdown valve may result in accelerated failure of the letdown valve.